Carrier 37HS specifications System-Powered Controls with Electric Interface

Fig. 67 Ð Variable Volume Controls Ð Minimum Flow: Thermostat Partially Open,

Controller Partially Open, Unit Delivering Minimum Flow

System-Powered Controls with Electric Interface

VAV COOLING WITH ELECTRIC WARM-UP Ð Another approach to warm-up is the addition of an electric warm-up valve; the operating principle is the same as system powered warm-up. A building with its conditioning system shut down during unoccupied hours must be brought close to operating temperature before the occupants arrive. Heated air supplied to the space from a central unit can quickly restore operating conditions. But because the VAV thermostat has closed the unit, the result of the lack of load, the system powered con- trol needs to be overcome.

An electric warm-up valve located on the Moduline unit between the volume controller and the thermostat is wired to the primary air source machine room. The valve can be activated by a simple manual or timer switch or it can

be connected to the warm-up terminal of the central station unit, if such is provided. Thus, when heated air be- gins to ¯ow in the distribution duct, the bellows is bled down by the action of the electric warm-up valve and heated air can ¯ow into the conditioned space (Fig. 68).

In the cooling mode (Fig. 69), air ¯ows freely between the volume controller and the thermostat and control func- tion is not affected. When warm-up is activated, the nor- mally open port closes and the control circuit is closed after the volume controller. As with system powered warm-up, this causes the pressure to rise in the low pressure chamber of the volume controller and opens the valve that bleeds air from the bellows pressure chamber. The increased bleed rate lowers the bellows pressure, opening the unit damper, and allowing the ¯ow of primary air to increase up to the con- stant volume set point of the volume controller.

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